1. Field of the Invention
The present invention relates to a diversity circuit, more specifically to a diversity circuit for antenna diversity of a radio communication system employing a spread spectrum system, such as code division multiple access (CDMA).
2. Description of the Related Art
In a conventional combining diversity circuit, a set of an antenna, receiver, and demodulator is needed for each of the diversity paths. Signals from each diversity path are combined at a final stage of signal processing by using the maximal ratio combining technique, where finally the signals from each diversity path are combined into one composite signal.
FIG. 2 illustrates an example of the foregoing conventional circuit. FIG. 2 is a typical example of an antenna combining diversity circuit. Signals received by each of two antennas 201, 202 are respectively demodulated into base band signals by receivers 203, 204. The demodulated outputs of receivers 203, 204 are respectively converted into digital signals by A/D converters 205, 206. The digitally converted signals are respectively demodulated by digital demodulators 207, 208, where finally the two demodulated signals are added, or combined by a combiner 209. The output from the combiner 209 is decoded by a decoder 210 to reproduce transmitted data.
In the conventional combining diversity circuit having dual branches, a set of an antenna, receiver, A/D converter and digital demodulator is needed for each of the two diversity paths, which doubles the scale of the circuit compared to a case without applying the diversity. Naturally, increased scale of the circuit will increase the power consumption, cost, and size of the unit, which makes it difficult to use the diversity circuit for a portable telephone, for example.